This invention relates to a method for thickening or stabilizing aqueous media and electrolyte-containing aqueous media comprising the addition to the media of neutrally-charged polyampholytes which are graft copolymers of polysaccharides with a zwitterionic monomer or cationic/anionic monomer pairs. These polyampholytes are characterized by water dispersibility and resistance to loss of intrinsic viscosity in the presence of electrolytes in aqueous media. This invention also relates to an inverse emulsion (water-in-oil) method for preparing these polyampholytes and to a novel polyampholyte composition prepared from guar gum that is useful as a thickener and stabilizer in electrolyte-containing aqueous media.
It is well known that the rheology of aqueous media may be modified and controlled by the addition of high molecular weight, water soluble polymers, such as polyacrylamide, polystyrene sulfonate, polysaccharides, carboxymethylcellulose, xanthan gum and guar gum, and their derivatives. It is also well known that the introduction of ionic substituent groups onto polysaccharides (e.g., starches) or other polymers improves the retention of water and the polysaccharide or polymer in applications such as papermaking and oil well drilling. The nature of the ionic substituent has a substantial effect on the rheology of the polyionic polymer. A typical polyelectrolyte is highly water soluble or dispersible. The viscosity of a typical polyelectrolyte (polyanion or polycation) decreases rapidly in the presence of electrolytes. The typical polyampholyte (containing anionic and cationic moieties) is soluble or dispersible in electrolyte containing solutions and insoluble or not dispersible in water and often increases in viscosity in the presence of electrolytes.
The improvements in rheology control obtained by introducing ionic substituents onto a polymer are lost or substantially diminished when a polyionic polymer is used in an electrolyte-containing medium, such as the high conductivity ("thick stock") medium which may be encountered in papermaking.
Similar problems have been observed in the presence of electrolytes when polyionic derivatives of water soluble polymers are used in oil drilling operations, water flood recovery of subterranean oil and other industrial applications. The enhanced thickening ability of synthetic polyionic polymers, such as ionic copolymers of polyacrylamide and polystyrene sulfonate, is greatly reduced by the presence of electrolytes such as calcium or sodium chloride, and magnesium sulfate in subterranean oil formations. These electrolytes are normally present in ground water and in drilling mud fluids. Therefore, the utility of these polymers in oil recovery is also greatly reduced.
Polymers known to be resistant to this electrolyte effect, such as xanthan gum, lack thermal or shear stability, or are easily biodegraded, too costly, or otherwise unacceptable for oil recovery operations.
Several patents address this oil recovery problem. For example, U.S. Pat. No. 4,222,881 to Byham, et al. discloses an amphoteric polyelectrolyte thickener which is a block copolymer of quaternary vinyl pyridinium sulfonate and alpha-olefin or hydrogenated diene (i.e., vinyl pyridinium sulfonate-styrene block copolymer), containing equimolar charge ratios. U.S. Pat. No. 4,673,716 to Siano, et al. discloses high molecular weight terpolymers of acrylamide, oil soluble higher alkylacrylamide and alkali metal acrylate which are capable of thickening water or brine.
Polymers useful in oil recovery, and in other acid, base or salt containing aqueous media, are disclosed in U.S. Pat. No. 4,710,555 to Peiffer, et al. Synthesized from acrylamide, sodium styrene sulfonate and methacrylamidopropyltrimethylammonium chloride, these polymers are claimed to have viscosity-polymer concentration relationships that are invarient with the presence of acid, base or salt and to possess a balance between conventional polyelectrolyte and polyampholyte behavior. Anionic and cationic moieties are not necessarily present in amounts that result in an equimolar charge ratio.
For oil drilling fluid applications, U.S. Pat. No. 4,600,515 to Gleason, et al. discloses high molecular weight, water-in-oil emulsion copolymers of acrylamide and a water soluble salt of acrylic acid which display improved divalent cation tolerance. U.S. Pat. No. 4,652,623 to Chen, et al. discloses an oil drilling polyampholyte synthesized from an unsaturated carboxylic acid, an unsaturated sulfonic acid, an unsaturated cationic-containing compound and a non-ionic monomer. U.S. Pat. No. 4,637,882 to Peiffer, et al. discloses drilling muds prepared from terpolymers based on N-vinyl-2-pyrrolidone/sodium styrene sulfonate/methacrylamidopropyltrimethylammonium chloride.
None of these references teach the use of polyamphoteric graft copolymers prepared from high molecular weight polysaccharides, such as starch, cellulose or guar gum, as electrolyte-tolerant thickeners or stabilizers. The polysaccharide graft copolymers of this invention are effective rheology control agents in the presence of electrolytes. Unlike conventional polyampholytes, they are soluble or dispersible in water. Such graft copolymers also offer the advantage over those disclosed in the references of economy, ease of preparation and ease of handling in the form disclosed herein.
The preparation of an electrolyte-tolerant polyampholyte from starch or hydroxyethyl cellulose by graft copolymerization with a water soluble cationic/anionic monomer pair and a neutrally-charged water soluble monomer has been reported. Salamone, J. C., et al., "Aqueous Salt Absorption by Ampholytic Polysaccharides," Polymer, 26: 1234-1238 (1985). These polysaccharide polyampholytes exhibited increasing viscosity with the addition of increasing amounts of sodium chloride in solution. They also exhibited increasing water absorbancy, particularly in the presence of electrolytes, when the percent incorporation of the ionic monomers onto the polyampholyte was increased.
It was noted by Salamone, et al., that use of cobalt-60 radiation to initiate graft copolymerization leads to tough, rubber-like copolymers, presumably caused by excessive cross-linking. Copolymers prepared by cobalt-60 initiation were not water soluble. Successful results were only reported using cerium (IV) initiation. (See also, Kao-Ching Lin, Hydrophilic, Ampholytic Graft Copolymers, M. S. Thesis, University of Lowell, September, 1983.) However, cerium (IV) initiation has the disadvantage that highly toxic material must be handled. Thus, there is a need for a method of graft polymerization of polysaccharides which may be carried out using another type of initiation.
Other disadvantages of known methods for preparing polysaccharide polyions, such as difficulty concentrating the product, isolating the polyampholyte from the reaction medium for various applications, or handling unstable reagents, have been addressed in the literature. For example, U.S. Pat. No. 4,017,460 to Tessler discloses a method for preparing amphoteric starch derivatives where the zwitterionic reagent is synthesized in situ from a secondary amine and an acid or ester of an acid. This method ameliorates problems associated with preparation and handling of unstable reagents. However, this reference concerns starch derivatives and does not address the problems associated with known methods for graft copolymerization of the polyampholytes of this invention.
Thus, it is an object of this invention to provide economical, commercially feasible solutions to the need for high molecular weight polyampholytes for thickening and stabilizing electrolyte-containing aqueous media.